1. Field of the Invention
The invention relates to a power laser delivery system for medical and industrial applications and more specifically to a laser delivery system which uses a graded index fiber manufactured and utilized to transmit a high level of energy without sustaining damage.
2. Information Disclosure Statement
Power laser delivery devices are becoming increasingly more important in medicine for effectively performing procedures ranging from tissue cutting, coagulating, welding and destruction of kidney stones to ablation of arterial plaques and in industry. The medical and industrial laser delivery systems generally employ optical fibers to carry laser radiation to remote targets. They provide freedom of movement and reliability unmatched by articulated arm laser delivery systems. They can handle high power. Their low cost makes them disposable. A possibility of optical image transmission through such fibers provides additional advantages in many medical and industrial applications.
Power laser delivery fiber systems essentially consist of an appropriate constant wave, cw, or pulsed laser source as for example CO.sub.2, Nd-YAG, or argon; one or more flexible optical fibers optically connected to the laser source by means of some input connector or some input optical focusing system. The fiber transmits the powerful laser radiation to its output end or handpiece.
High power lasers generally radiate a multimode beam whose traverse modes can be approximately described with the help of Gauss-Laguerre functions. For maximum delivery of laser power, it is thus desirable to have a fiber which can transmit all the generated Gauss-Laguerre laser modes. It is further known that scattering of the powerful laser beam in the fiber material and different nonlinear effects in the fiber result in destruction of the laser beam and in damage to the fiber. Generally the damage to the fiber first occurs in the vicinity of core-cladding boundaries because beam power density increases at such interfaces due to constructive interference of the incident and reflected beams and also due to defects arising from the presence of dissimilar materials in this region.
Step-index multimode fiber is not very suitable for transmission of the powerful laser beams, since the main guiding method for these fibers is the interaction of the beam with the core-cladding boundary. Step-index fibers also do not preserve well all the desired beam properties. Although power losses in such fibers may be small, image transmission is not possible with such fibers. Finally the mode structure in such fibers is not matched to the Gauss-Laguerre mode configuration of the laser beam.
Employing graded-index optical fibers ameliorates some of these problems. Traverse variation of the refractive index across the core radius provides the means for guiding the laser beam confining the beam to its longitudinal axis, due to the refractive index profile, rather than due to interaction with the core-cladding boundary. The graded-index optical fibers can provide an image transmission. Moreover, the modes in a graded-index fiber with a parabolic traverse distribution of its refractive index are described by Gauss-Laguerre functions. With effective coupling of a Gauss-Laguerre laser beam into such fibers, the laser's mode structure is preserved, providing maximum delivery of the laser energy.
Transmission of powerful laser beams through graded-index fiber with the normal exact parabolic index profile still has some disadvantages. The parabolic index profile is close to an optimum profile, having an equally spaced spectrum of the mode propagation constants. The parabolic index profile results in almost ideal focusing of all the beam rays. As a consequence of this focusing, any incident laser beam with a regular, planar or spherical, initial wavefront will exhibit a strong field localization in the fiber, especially at the first few focal regions. This leads to an enormous increase of the beam power density in these focal regions and causes different nonlinear effects, fiber damage etc.
European Patent EP 438 653 A2 offers a flexible graded-index optical fiber for transmitting the powerful laser beams with preservation of their mode structure. The fiber is optimized by combining the advantages of graded-index and step-index fibers in such a way as to provide a transmission of the Gauss-Laguerre laser beam through the fiber with preservation of its mode structure. Merely the preservation of the beam mode structure is not sufficient for effective operation of fibers in powerful laser delivery systems. In the general case of fiber excitation with an arbitrary input beam, the high power densities in the focal regions remain a problem.
Another attempted solution is a graded-index fiber with a non-optimum refractive index profile which would destroy the periodically repeating prescribed phase relations between fiber modes. The phases of different modes coming to the output end face of the fiber are equally distributed over the range of phases from 0 to 2.pi.. Interference of all the fiber modes with such distribution of phases results in a complicated speckle pattern at the output end face of the fiber. The angular extent of the output beam is greatly increased and image transmission through such a fiber is generally impossible. In these respects such fibers are not much better than step-index fibers. Generally in most laser power transmission systems, not only the absolute power transmitted, but also the achievable power density after transmission through the fiber is of key importance. Image transmission, that is focused transmission, is also required.